Force Regen­er­ate Thumb­nails will process all your exist­ing image attach­ments and will recre­ate all of the need­ed thumb­nails sizes by the cur­rent theme, while delet­ing the old ones so you don’t waste host­ing space; very help­ful when switch­ing themes: http://wordpress.org/plugins/force-regenerate-thumbnails/

Moving WordPress sites

Dupli­ca­tor — this is one of the eas­i­est and most help­ful WP trans­fer plu­g­ins out there; you sim­ply back­up your entire Word­Press instal­la­tion, data­base includ­ed, download/upload the result­ing files, and run a sim­ple php installer that will recre­ate the pre­vi­ous instal­la­tion on your new host­ing, domain, or what­ev­er: http://wordpress.org/plugins/duplicator/